a(-3) = a(-2) = a(-1) = 0, a(0) = 1, a(n) = a(n-1) + 2*a(n-2) + 2*a(n-3) + a(n-4), for n>0.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =3a(6) =7a(7) =16a(8) =37a(9) =86a(10) =199a(11) =461a(12) =1068a(13) =2474a(14) =5731a(15) =13276a(16) =30754a(17) =71242a(18) =165033a(19) =382301a(20) =885605a(21) =2051515a(22) =4752360a(23) =11008901a(24) =25502256a(25) =59076293a(26) =136850967a(27) =317016966a(28) =734373742a(29) =1701185901